Fuel cells convert a fuel into usable electricity via electrochemical reactions. A significant benefit to such an energy-producing means is that it is achieved without reliance upon combustion as an intermediate step. As such, fuel cells have several environmental advantages over internal combustion engines and related power-generating sources. In PEM fuel cells, which may include a proton exchange membrane or a polymer electrolyte membrane, for example, two catalytic electrodes are separated by an ion-transmissive medium (such as Nafion™) in what is commonly referred to as a membrane electrode assembly (MEA). The electrochemical reaction may involve oxidation of hydrogen molecules (H2) at an anode to generate two protons (H+) and two electrons and reduction of an oxidizing agent such as oxygen (O2) at a cathode to form water. The protons generated at the anode may pass through the ion-transmissive medium to combine with the oxidizing agent, and the electrons generated at the anode may be diverted as direct current (DC) through an external electric circuit before being directed back to the cathode to participate in the reduction of the oxidizing agent. The external electric circuit typically includes a load where useful work may be performed. The power generation produced by this flow of DC electricity can be increased by combining numerous such cells to form a fuel cell stack.
The use of noble-metal catalysts can significantly add to the overall cost of a fuel cell system. More efficient dispersions of platinum and related catalyst materials has led to lower platinum loadings and related lower overall fuel cell system cost. Nevertheless, performance and durability issues become even more acute, as low platinum catalyst loading leads to a low overall reactive surface area that is more sensitive to surface contaminants that cause reversible performance loss. In addition, electrodes with low roughness factors, i.e., those with low ratios of active area to geometric area) have increased sensitivity to ionomer adsorption. Ionomer adsorption increases local resistance of oxygen transport to active catalyst sites because anions and organic molecules block surface sites on the platinum catalyst. Therefore, preventing the adsorptions both of anions and ionomers becomes imperative for enabling state-of-art electrodes with low platinum loading to function in systems such as automotive fuel cell systems. In the present context, durability is the ability of the platinum electrode to avoid or eliminate reversible voltage degradation, particularly the voltage degradation that may occur from adsorption of anions, ionomers, or both.
The oxygen reduction reaction (ORR) presents a challenge, because any suitable ORR catalyst must satisfy two competing objectives. First, the catalyst should minimize adsorption of spectator species that may poison the catalyst. Second, the catalyst must be sufficiently catalytically active to conduct oxygen reduction at potentials as close as possible to the roughly 1.2 V ORR reversible potential. Achieving both objectives is especially difficult when platinum loading is low.
Therefore, there remain ongoing needs for membrane electrode assemblies and fuel-cell systems having electrocatalysts that retain activity even with low levels of catalyst loading. There also remain ongoing needs for methods of preparing such membrane electrode assemblies and incorporating them into fuel-cell systems.